Air Conditioner Not Cooling? Check These First

Quick answer

When an air conditioner is not cooling, the most common causes are a thermostat set to the wrong mode, a dirty air filter, closed or blocked vents, or ice on the coils. Check the settings and the filter first. This guide is for beginners with a window unit, a portable unit, or central air who want safe checks before calling a qualified technician.

Safety first

• Unplug a window or portable unit, or switch off its breaker if you can’t reach the plug safely, before removing the filter or cleaning.
• For central air, turn the system off at the thermostat. Before clearing debris around the outdoor unit, switch off power at the outdoor disconnect (the switch box mounted near the unit) or the breaker.
• Never remove panels or covers, touch wiring or electrical parts, or straighten or clean coils hidden behind panels.
• Problems with the sealed cooling system, including anything to do with refrigerant, are only for a licensed professional.
• If you smell burning, see sparks, or notice water near outlets or electrical parts, stop. Don’t touch the unit or its switches if water is near electrics; switch off the breaker if you can reach it safely and call a professional.

Before you begin

Confirm the unit has power: the display or fan should come on. For a plug-in unit, many power cords have a built-in safety plug with Test and Reset buttons; if it has tripped, press Reset as your manual describes. Check whether a breaker or GFCI outlet (the kind with Test and Reset buttons) has popped.

Find the model number on the rating label so you can read the manual. If you’re not sure where to look, see how to find your appliance model number. Error codes and display messages vary by brand and model, so look any code up in the manual or on the manufacturer’s support site.

Possible causes

Things you can check yourself:

• Wrong thermostat or remote settings. “Fan” or “Dry” mode moves air without cooling; a set temperature above room temperature won’t call for cooling.
• Dirty air filter. A clogged filter cuts airflow and can lead to ice on the coils.
• Closed or blocked vents. Furniture, rugs or curtains over registers (the grilles where air comes in or out) restrict airflow.
• Frozen coils. Ice forms when airflow is poor; the unit blows weak, warm air.
• Debris around the outdoor unit. Leaves, grass or a cover left on can block the airflow it needs.
• Unit too small for the room, or heat sources nearby. A sunny window or a crowded room can overwhelm a small window unit.

Problems that need a technician:

• A refrigerant leak or other sealed-system fault, which needs a licensed professional
• Electrical faults, including parts that help the motor start
• A failed fan motor or outdoor unit that won’t run
• Dirty or damaged coils behind panels
• Error codes that keep returning

Step-by-step checks when the AC isn’t cooling

1. Check the mode and temperature. Set the thermostat or remote to “Cool” and choose a temperature a few degrees below the current room temperature. For central air, the fan setting can be “Auto.” If the thermostat runs on batteries and the screen is blank or faint, replace them. Does the unit start cooling within several minutes? If yes, you’re done. If not, go to step 2.
2. Check the air filter. Turn the unit off and unplug it (or switch off the system). Slide out the filter and hold it up to light. If you can’t see light through it, clean or replace it as your manual describes. See how to clean or replace an air conditioner filter. Is airflow stronger after the filter is clean? If yes, give it some time to cool the room.
3. Open and clear the vents. For central air, open supply registers in the rooms you’re cooling and make sure return grilles aren’t blocked. For a window or portable unit, keep curtains and furniture away from the front, and for a portable unit make sure the exhaust hose is straight and attached to the window kit.
4. Look for ice. Look at the front of a window unit with the filter out, or at the indoor unit area you can see without removing any panels for central air. Do you see frost or ice? If yes, turn cooling off and run only the fan (or turn the unit off) so the ice melts, which can take a few hours or longer. Don’t chip at the ice. After it thaws, put in a clean filter and try cooling again. If ice returns, call a technician.
5. Clear around the outdoor unit (central air). Switch off power at the outdoor disconnect or the breaker. Remove any cover, and pick up leaves, grass clippings and debris around the unit by hand. Keep plants trimmed back as your manual suggests. Don’t open the unit or bend its fins. Restore power and try again.
6. Reduce the heat load. Close blinds on sunny windows, keep doors to the cooled room shut, and avoid running the oven or dryer during the hottest hours. Is the room slowly getting cooler? If yes, the unit may simply be working hard in very hot weather.

When to stop and call a qualified technician

Contact a qualified HVAC or appliance technician, or the manufacturer’s service network, if:

• The unit still blows warm air after you’ve checked settings, the filter, vents and debris
• Ice keeps forming after you’ve let it thaw and replaced the filter
• The outdoor unit doesn’t run, hums, or makes grinding or clicking noises
• The breaker trips again after you reset it
• You notice a burning smell, sparks, or water near electrical parts
• An error code keeps coming back after you’ve looked it up

Anything involving the sealed system, refrigerant or wiring needs a licensed professional.

FAQ

Why is my AC running but not cooling the house?

Common reasons are a dirty filter, the thermostat on “Fan” instead of “Cool,” blocked vents, or ice on the coils. Check those first. If the air is still warm, there may be a problem only a technician can diagnose.

How long does a frozen AC take to thaw?

It depends on how much ice there is and the room temperature, but it often takes several hours. Turn cooling off and let it melt on its own; running only the fan can help. Place towels nearby in case of extra dripping.

Does a dirty filter really stop an AC from cooling?

Yes. A clogged filter restricts airflow, so less air passes over the cold coil and the coil can freeze. The ENERGY STAR maintenance checklist recommends checking filters regularly.

Can I recharge or “top up” my AC myself?

No. Adding or handling refrigerant is a job for a licensed professional, because the sealed system needs special tools and training. If you suspect a leak, call a technician.

Where can I find more air conditioner help?

The air conditioners section has guides on leaks and filter care. Your manual also lists maintenance steps for your model.